1. Field of the Invention
Embodiments of the invention described herein pertain to the field of multi-stage centrifugal pumps. More particularly, but not by way of limitation, one or more embodiments of the invention enable a centrifugal pump for handling abrasive-laden fluid.
2. Description of the Related Art
Fluid, such as gas, oil or water, is often located in underground formations. In such situations, the fluid must be pumped to the surface so that it can be collected, separated, refined, distributed and/or sold. Centrifugal pumps are typically used in electric submersible pump (ESP) applications for lifting well fluid to the surface. Centrifugal pumps impart energy to a fluid by accelerating the fluid through a rotating impeller paired with a stationary diffuser. A rotating shaft runs through the central hub of the impeller and diffuser. A motor upstream of the pump turns the shaft, and the impeller is keyed to the shaft, causing the impeller to rotate with the shaft.
Each rotating impeller and stationary diffuser pair is called a “stage”. The impeller's rotation confers angular momentum to the fluid passing through the pump. The angular momentum converts kinetic energy into pressure, thereby raising the pressure on the fluid and lifting it to the surface. Multiple stages of impeller and diffuser pairs may be used to further increase the pressure lift. The stages are stacked in series around the pump's shaft, with each successive impeller sitting on a diffuser of the previous stage.
FIG. 1 illustrates a conventional impeller of the prior art. As shown in FIG. 1, impellers typically have a conventional skirt 100 extending axially on the bottom of the impeller. The conventional skirt 100 wear ring rotates inside the conventional diffuser exit skirt 105. The close conventional clearance 1 between conventional impeller skirt 100 and the conventional diffuser exit skirt 105 provides a hydraulic seal to restrict fluid from leaking back to the eye of the impeller when fluid is pumped. The hydraulic seal helps to increase volumetric efficiency, maintain desired performance and assist with radial stabilization.
During operation of the pump, abrasives such as sand, dirt and other solid particles in the pumped fluid pass through clearance 1 between the conventional impeller skirt 100 and conventional diffuser exit skirt 105, wearing down those pump components. As the skirt wears, the gap increases, fluid and pressure leaks, and the pump performance is reduced. The conventional clearance 1 between conventional impeller skirt 100 and conventional diffuser exit skirt 105 should be between about 0.010 inches and 0.014 inches diametrically (0.005-0.007 inches radially), depending upon the size of the pump. Gaps in excess of about 0.022 inches diametrically cause reduced pump production, which may necessitate that the pump be pulled out of operation.
Impellers also have a conventional balance ring 115 extending axially on the top side of the impeller. Conventional impeller balance ring 115 rotates inside the conventional diffuser inlet 120. There is also a close conventional clearance 2 between conventional impeller balance ring 115 and conventional diffuser inlet 120. During operation of the pump, the hydraulic seal which forms within the space between conventional balance ring 115 and conventional diffuser inlet 120 provides radial support to the pump. Conventional balance holes 125 drilled in the top of the impeller may be included to regulate the downthrust force.
Abrasives in pumped well fluid flow through conventional clearance 2 during pump operation, wearing down the conventional balance ring 115 and conventional diffuser inlet 120. This abrasive wear increases the conventional clearance 2 between the conventional balance ring 115 and conventional diffuser inlet 120, and in such instances, radial support decreases and pump performance degrades. The lack of radial support increases wear. Performance degradation may cause an ESP system to fail because of the lack of lift.
Conventionally, a hard coating such as nickel nitride, has been applied to impeller skirts and balance rings in order to prevent wear from abrasives in well fluid. However, coating an impeller is time consuming and expensive.
As is apparent from the above, current centrifugal pumps are not well-suited to handling abrasives. Therefore, there is a need for a centrifugal pump for handling abrasive-laden fluid.